B-1 cells constitute a unique population of B cells, with numerous distinguishing phenotypic, transcriptomic, and functional characteristics. B-1 cells differ from conventional B-2 cells in the rules that govern stimulated cell cycle responses and in the unstimulated, spontaneous secretion of non-immune immunoglobulin. The long term objectives of the proposed work are twofold: 1) to elucidate pdmary characteristics that dictate the unique nature of B-1 cells; 2) to identify key mediators that specify proliferative responses in, and differentiative features of, B-1 cells. The proposed studies build on a longstanding collaboration between the two project leaders that has already produced new insights regarding previously unrecognized restriction points regulating cell cycle progression in B cells. The proposed study will exploit these advances to further probe the nature of cell cycle regulation, and at the same time take advantage of the proven complimentary expertise of the investigators, and recent key findings, to extend this work to elucidate the determinants of immunoglobulin secretion. Several molecular approaches will be utilized to address the function of cyclin D3-cdk4 complexes in B cells (Project 1) and relationship between Bcl-6 and immunoglobulin secretion, and !the role of the peritoneal environment, in B-1 cells (Project 2). These studies will generate new information regarding the underlying nature of B-1 cells, and will identify means to manipulate B cell clonal expansion and spontaneous immunoglobulin secretion to improve resistance to infection and/or intoxication by natural microorganisms or bioterrorism agents, and to forestall and/or treat autoimmune disease.